1. Field of the Invention
This invention relates to a light sailboat having full lateral and fore-aft symmetry and, more particularly, to one using a swingable seat boom to balance the sail force moment.
2. Description of the Prior Art
The lateral equilibrium of a sailboat is largely determined by a balance between two force couples, i.e. between two pairs of equal and opposite, but offset, forces. The first of these force couples comprises the components in the lateral plane of the wind force on the sail and the water force on the centerboard. The second couple comprises the upward lift on the hull (typically a combination of buoyancy and dynamic forces) and the downward force of gravity acting on boat and crew. The sailboat is in lateral equilibrium when the moments exerted by these two force couples are equal in magnitude and opposite in sense.
Designers of light sailboats strive to enlarge the offset distance between lift and gravity as much as possible so that for a given weight a large sail can be used. One way to do this is to provide a wide hull so that heeling will shift the lift force far to the lee side. Another similar way is to provide two (or even three) widely spaced narrow hulls. Boats with such hulls, however, sacrifice maneuverability for speed. In particular, they come about (i.e. make upwind turns) slowly and uncertainly because their small inertia is quickly absorbed by their resistance to turning. The proa, a type of multihull boat with fore-aft but not lateral symmetry, overcomes this problem by turning in a different way (called shunting) that does not depend on inertia.
Although proa-like boats can make upwind turns by shunting, they must sometimes execute a cumbersome maneuver to make a downwind turn. They can turn directly away from a downwind course in only one direction. To turn the other way, they must first turn away from the desired direction, then shunt. Conventional sailboats with lateral symmetry must also execute a special maneuver (called jibing) on one of the two downwind turns. This maneuver, while not slow, is somewhat awkward and can even be dangerous if not executed skillfully. For very fast sailboats downwind turns are important because such boats find it advantageous to tack downwind.
A solution to the downwind turning problem for a sailboat having the symmetry of the proa is offered by Pavincic (U.S. Pat. No. 4,273,060). That solution, however, depends on the use of a circular hull, which has much greater drag for a given displacement than an elongated hull. The boat set forth in the present disclosure permits direct downwind turns in either direction without shunting or jibing and without compromising the low drag of the hull.
The symmetry of the design disclosed in this specification, however, poses special problems not encountered in other designs. One of these is that the sail must be supported in such a way that it can assume any angle about the vertical axis. Conventional support structures tend to severely limit either the sail's size or the angle through which it can turn. A tripod support that apparently would allow complete rotation of a sail is disclosed by Jamieson in U.S. Pat. No. 4,044,702. That structure, however, appears to put a relatively large mass aloft and to generate a relatively large parasitic drag. It also requires a very wide base and is, therefore, unsuitable for the single hull boat disclosed herein.
Other less relevant patents that disclose sailboats having the symmetry of the proa include U.S. Pat. Nos. 2,756,711 (Simpson); 3,094,961 (Smith); 3,173,395 (Laurent); 3,223,065 (Wilson); 3,295,487 (Smith); 3,304,899 (Weatherly); 3,336,890 (Laurent); and 3,985,090 (Rineman).
An alternative to wide or multiple hulls to achieve a large lever arm between lift and gravity is provided by devices such as hiking straps and seat booms for shifting the crew weight outboard. The invention disclosed herein uses a seat boom. Different kinds of prior art seat booms are discussed, for example, in U.S. Pat. Nos. 1,885,247 (Fox) and 4,539,926 (Boffer). Another feature of the invention disclosed herein is a horizontally pivoted sail mount that allows the foot of the sail to swing outward and upward under wind pressure A relevant sail is disclosed in the cited patent of Fox (U.S. Pat. No. 1,885,247). This patent, however, does not suggest a nonlinear mechanical connection between a sail and a seat boom that constitutes the balancing mechanism set forth in the present disclosure.